bonding: support encapsulated ipv6 TSO

If using a sixtofour device on top of a bonding device,
skb segmentation of TCP traffic is done right before calling
bonding xmit, because bonding only enables TSO for IPv4.

This patch improves single flow performance by about 120 % on my hosts,
because segmentation is deferred right before calling slave xmit.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>